Is the spectral response of
the microbolometer array 7.5µ to 13.5µ, or is the spectral response a
function of the lens?

The spectral response of the microbolometer is influenced by
all of the optical interfaces between the focal plane array and the
scene or object being imaged. These include the sensor vacuum package
front window, the lens, any protective window in front of the lens, and
the atmospheric transmission between the object and the camera. FLIR`s
standard sensor packages have front windows that define the short
wavelength cut-on of 7.5μ, while the bolometer itself defines the long
wavelength cut-off of 13.5μ, assuming negligible atmospheric
attenuation.